1. Field of the Invention
The present invention relates to a planographic printing plate precursor and a printing method for a planographic printing plate using the same. More particularly, the present invention relates to a planographic printing plate precursor that can be used for direct plate-making by scanning the plate with an infrared laser on the basis of digital signals from a computer, or the like, and to a planographic printing method using the planographic printing plate precursor wherein the printing plate can be developed on a printing machine.
2. Description of the Related Art
Generally, a planographic printing plate is formed of lipophilic image portions which receive ink during printing, and hydrophilic non-image portions which receive dampening water.
Planographic printing utilizes a property of water and oil based ink repelling each other, wherein the lipophilic image portions are used as ink receiving areas and the hydrophilic non-image portions are used as dampening water receiving areas (non-ink-receiving areas). In this printing method, ink is thinly deposited on only the image portions, and then the ink is transferred onto a medium which is printed, such as paper, to print the image. Conventionally, a PS plate provided with a lipophilic photosensitive resin layer on a hydrophilic substrate thereof has been widely used. In a plate-making method for the PS plate, usually, a planographic printing plate precursor is exposed through an original image such as a lith film. Thereafter, the photosensitive layer is left at the image portions, and that at the non-image portions are dissolved and removed using a developing solution to expose the surface of the aluminum substrate. Thus, a desired printing plate is obtained.
In the conventional plate-making process for the PS plate, after the exposure, a process which dissolves and removes the photosensitive layer at the non-image portions using a developing solution, or the like, which is suitable for the photosensitive layer, is necessary. One task is to eliminate or simplify this additional wet type treatment. Particularly in recent years, with consideration of the global environment, disposal of waste liquid produced during the wet type treatment is a great concern of the whole industrial world. Therefore, there is an increasing demand for improvements in this regard.
As a simple plate-making method corresponding to these demands, a method using a photosensitive layer, which is able to be removed from the non-image portions of the printing plate precursor in a usual printing process, wherein, after being exposed with light, the printing plate is developed on a printing machine to obtain a final printing plate, has been proposed. This plate-making method for a planographic printing plate is called on-machine development. Specifically, the method includes, for example, use of a photosensitive layer which is soluble in dampening water or a solvent for ink, mechanical removal of the photosensitive layer at the non-image portions by contacting them with an impression cylinder or a blanket cylinder in the printing machine, or the like. However, since an image formed in the photosensitive layer is not fixed until it is developed after exposure in conventional image recording methods, which utilize ultra violet or visible light, handling of the exposed printing plate for on-machine developing is troublesome because the printing plate has to be completely shielded from light and stored at a constant temperature before it is set in a printing machine.
In this field in recent years, digitized techniques which electronically process, store and output image information using a computer have widely spread, and various types of newly developed image output techniques for use with such digitized techniques have been put into practice. Accompanying this, a computer-to-plate technique, in which a printing plate precursor is scan-exposed with highly convergent radiation, such as a laser beam, which is modified based on digitized image information to directly produce a printing plate without using a lith film, has been attracting attention. Along with this, it has become technically important to obtain a printing plate precursor which is suitable for this purpose.
Therefore, a simplification of plate-making process and an introduction of dry-type processing are more strongly desired than in the past from the above-described environmental point of view and necessity for adaptation to the digitized techniques.
Since high-output semiconductor lasers or solid state lasers such as a YAG laser are now available at low prices, particularly, a plate-making method which employs such lasers as image recording means has been regarded as a favorable method for producing a printing plate by scan-exposure which can be readily incorporated into the digitized techniques. In a conventional plate-making method, image recording is carried out by performing imagewise exposure onto a photosensitive printing plate precursor with low- to mid-level illumination, thereby causing an imagewise change in physical properties of the surface of the printing plate precursor by a photo-chemical reaction. In a method using high-power-density exposure employing a high-output laser, areas to be exposed are irradiated by a large quantity of concentrated light energy for a very short time and the light energy is efficiently converted into thermal energy. The heat is used to cause a change such as a chemical change, a phase change, a change in form or structure, or the like, and the change is utilized for image recording. That is, image information is input by light energy such as a laser beam, and an image is recorded by a reaction caused by thermal energy. Usually, such a recording method utilizing heat generated by the high-power-density exposure is called heat-mode recording, and conversion of light energy into thermal energy is called photo-thermal conversion.
A major advantage of a plate-making method utilizing heat-mode recording means is that a photosensitive material used for the heat-mode recording is not sensitive to light at normal illumination levels such as room light, and an image recorded by high-illumination exposure is not necessarily fixed. That is, when a heat-mode photosensitive material is used for recording an image, it is insensitive to room light before exposure, and fixing of the image after exposure is not essential. Therefore, for example, if a photosensitive layer which is rendered insoluble or soluble by heat-mode exposure is used, and a process for producing a printing plate by removing imagewise the exposed photosensitive layer is carried out in a manner of on-machine development, it is possible to provide a printing system in which an image is not affected even if the plate is exposed to ambient light for a certain time after image exposure during development, namely, removal of non-image portions. Therefore, by using heat-mode recording, a planographic printing plate precursor which is desirable for on-machine development is expected to be obtained.
Progress in laser technology has been remarkable in recent years, and high-output and small solid state lasers and semiconductor lasers, particularly those that emit an infrared ray in a wavelength range from 760 nm to 1200 nm, are readily available. These lasers are very useful as a light source for recording used for plate making directly from digital data from a computer, or the like. However, since the majority of photosensitive recording materials which are useful in practice have sensitivity to visible light having a wavelength of 760 nm or less, images cannot be recorded on them with an infrared laser. Therefore, a material which can be used for recording with an infrared laser is desired.
As an image recording material which can be used for recording with an infrared laser, a recording material comprising an infrared absorbent, an acid generator, a resol resin and a novolak resin is described in U.S. Pat. No. 5,340,699. However, for forming an image on such a negative-type image recording material, a heat treatment is required after exposure with a laser. Therefore, a negative-type image recording material which does not require a heat treatment after exposure has been desired.
For example, a recording material which comprises a cyanine dye having a certain structure, an iodonium salt and an addition-polymerizable compound having ethylenic unsaturated double bond, and which does not require heat treatment after imagewise exposure is described in Japanese Patent Application Publication (JP-B) No. 7-103171. However, this image recording material has a problem that strength of formed image portions thereof is low, thus if it is used as a planographic printing plate, the number of resulting prints which are acceptable is small.
A planographic printing plate precursor comprising a photosensitive layer provided on a hydrophilic substrate, which photosensitive layer contains fine particles of a thermoplastic hydrophobic polymer dispersed in a hydrophilic binder polymer, is disclosed in Japanese Patent No. 2,938,397. According to this patent, the planographic printing plate precursor is exposed with an infrared laser to form an image by coalescing the fine particles of the thermoplastic hydrophobic polymer with heat. Thereafter, the plate is set on a cylinder of a printing machine, and the plate can be developed on the machine using dampening water and/or ink. Although such a method for forming an image by coalescing the fine particles by simple thermal fusing exhibits good on-machine developability, strength of the image is low, and therefore there is a problem of insufficient plate-wear resistance.